A jack is generally configured with an oil return mechanism. A conventional oil return mechanism includes a liquid storing cavity, a liquid returning cavity, and a communicating cavity communicating between the liquid storing cavity and the liquid returning cavity. The communicating cavity is configured with a control switch for selectively conducting the on-off control of the communicating cavity. A speed of a fluid flowing from the liquid returning cavity to the communicating cavity has a direct impact on a descending speed of the jack in operation. However, when the control switch is on, the speed of the fluid flowing from the liquid returning cavity to the communicating cavity remains stable, and thus the jack descends rapidly either in a no load condition or in a load condition, which may result in accidents, for instance, a lifting load rapidly falling down from the operating jack to hurt an operator. Therefore, the conventional oil return mechanism is configured with a speed regulating device for regulating the speed of the fluid flowing from the liquid returning cavity to the communicating cavity in accordance with the different operation condition of the jack. However, the expensive speed regulating device adds significant cost to the oil return mechanism, and thus the oil return mechanism fails to satisfy industrial require's.